docs(textual): custom `Msg` renderers

[amaury1093]

Description

As discussed during our syncs, we want to allow app developers to create their own custom Msg renderers.

Includes also some smaller fixes:

• use RFC3339 for timestamps
• remove "weeks" from durations

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Author Checklist

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I have...

• [ ] included the correct type prefix in the PR title
• [ ] added ! to the type prefix if API or client breaking change
• [ ] targeted the correct branch (see PR Targeting)
• [ ] provided a link to the relevant issue or specification
• [ ] followed the guidelines for building modules
• [ ] included the necessary unit and integration tests
• [ ] added a changelog entry to CHANGELOG.md
• [ ] included comments for documenting Go code
• [ ] updated the relevant documentation or specification
• [ ] reviewed "Files changed" and left comments if necessary
• [ ] confirmed all CI checks have passed

Reviewers Checklist

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I have...

• [ ] confirmed the correct type prefix in the PR title
• [ ] confirmed ! in the type prefix if API or client breaking change
• [ ] confirmed all author checklist items have been addressed
• [ ] reviewed state machine logic
• [ ] reviewed API design and naming
• [ ] reviewed documentation is accurate
• [ ] reviewed tests and test coverage
• [ ] manually tested (if applicable)

[aaronc]

This is docs not chore

[amaury1093]

There's a scope creep in this PR, and the conversations are getting hard to follow. I'll remove all changes about tx bytes signing from this PR, please head over to https://github.com/cosmos/cosmos-sdk/pull/12910 if you're interested.

Let's keep this PR only about custom Msg renderers.

One piece left to discuss is about state reads: https://github.com/cosmos/cosmos-sdk/pull/12785#discussion_r943607392

[amaury1093]

All review comments have been addressed. @JimLarson @webmaster128 any last changes before I can get a approval from you?

[tac0turtle]

@amaurym do you need anything from the team here

[amaury1093]

@JimLarson I recall during our last call you'd said you would make some comments or commits to this PR. Still okay to do that?